1. Field of the Invention
The present invention relates to power amplifiers, and more particularly to a power amplifier that amplifies power of an input signal by using an output signal fed back.
2. Description of the Background Art
A conventional power amplifier disclosed in Japanese Patent Publication No. 61-23687 will now be described while referring to the block diagram of FIG. 7. In FIG. 7, the power amplifier includes an input terminal 1, a voltage amplifier stage 2, a power amplifier stage 3, including transistors Q1 to Q4, a feedback circuit 4, a DC power source 5, and an output terminal 6. Such a power amplifier operates as follows. An input signal of this power amplifier is provided to the voltage amplifier stage 2 through the input terminal 1. The voltage amplifier stage 2 amplifies the voltage of the input signal up to a given value on the basis of the feedback voltage described later and provides the output to the power amplifier stage 3. Since the power amplifier stage 3 is generally formed of a complementary circuit (SEPP circuit), its voltage amplification is one. Accordingly, when receiving the output signal from the voltage amplifier stage 2, the power amplifier stage 3 amplifies the current of the input signal up to a given value, without amplifying the voltage value of the input signal. The signal amplified by the power amplifier stage 3 is supplied to a load (not shown) through the output terminal 6, and is also supplied to the feedback circuit 4. The feedback circuit 4, having two internal resistors, attenuates the voltage of the supplied signal and feeds the feedback voltage back to the voltage amplifier stage 2.
As described above, with the combination of the voltage amplifier stage 2 and the power amplifier stage 3, the power amplifier amplifies power of the input signal and supplies it to the load.
The DC power source 5 supplies DC power-supply voltages + Vcc2 and -Vcc2 to the voltage amplifier stage 2 and the power amplifier stage 3. It is required to greatly vary the load voltage for supplying large power to the load since the power amplifier stage 3 has a voltage amplification equal to one. According to the power amplifier of FIG. 7, while the voltage supplied to the load must be varied up to the DC power-supply voltages .+-.Vcc2, the output voltage of the voltage amplifier stage 2 must also be varied up to the DC power-supply voltages .+-.Vcc2, since the voltage amplification of the power amplifier stage 3 is one. This requires the voltage amplifier stage 2 to be able to withstand the same voltage as the power amplifier stage 3. That is to say, in order for the power, amplifier to provide high power requires the use of transistors capable of withstanding high voltages in the voltage amplifier stage 2. However, transistors with high withstand voltages are expensive, which will result in an increased price of the power amplifier.